Why is my mirrored object in blender lighter/darker than the original object? Even with a simple cube?
Figured it out! I just hollowed out part of the mesh, on the side that would touch the mirrored object. 

And basically just went, delete / faces on that part. 

Hopefully this can help others having the same problem.
Here is an image showing what I mean. 

http://imgur.com/a/7PIHf